Well, you have Norman Osborn leading the Six. Plus, Kraven, Vulture, Electro and don't forget Sandman.
That's five. Five characters you could assemble by part Six. So who's a possible candidate for the sixth slot? I think Doc Ock could come back.
Now, Doc Ock's always traditionally been the leader of the Sinister Six, so it'd be great to have him on the team in some capacity. I'd love to see him and Osborn trade insults and argue about who could lead the team better. Osborn as leader and Ock as head scientist is something we've seen before in Ultimate Spider-Man comics and the Spectacular Spider-Man show.
But how do you bring back Doctor Octopus, on top of already bringing back Norman Osborn? He died and redeemed himself right before, too. Well it could work, especially if Osborn's been alive the entire time, plotting behind the scenes. Imagine at the end of Spider-Man 2, while Peter and MJ swing away from drowning Doc, Osborn flies a glider right into the ocean and fishes him out. Ock's lost some memory and gets some brain damage from almost drowning. So Norman does what he does best, he brain-washes and manipulates Ock into thinking it's Spidey's fault he's the monster he is and Spidey's fault his wife died. And since Ock comes back, he could be the character who rebells against Osborn and essentially helps Peter defeat him and the Sinister Six in Spider-Man 6.
But this is a Willem Dafoe thread and not a bring back Doctor Octopus thread, so my basic point is this: it is possible to assemble the Sinister Six by Spider-Man 6.